The present invention is directed to an apparatus and method for producing a beam of metastable atoms or molecules, and in particular, a system and method for producing a beam of metastable species for use in ionizing sample substances undergoing analysis by mass spectroscopy or other devices requiring ionization or excitation of substances.
Mass spectrometers are well known systems used for the detection and identification of chemical structures and quantitative elemental analysis of substances. In all known mass spectrometry methods, atoms or molecules to be sampled are excited and ionized, so as to create an ion beam. The ion beam is then accelerated through electric and magnetic fields to an ion collector, with the ion collector typically attached to an electrometer. The electrometer then translates signals received from the ion collector into a mass spectrum, which serves to indicate what elements (or radicals or fragments) are contained within the sample.
Many techniques have been suggested to excite and ionize the sample molecules and to fragment the ions from these molecules. These include the use of electrons to bombard species present in the gas phase, such as electron ionization; proton transfer reactions, such as those used in chemical ionization; or photoionization with lasers or other intense light sources. More recently, ionization has been accomplished by the use of metastable atom bombardment, in a which a neutral metastable species is used to bombard the sample molecules and fragment ions from these molecules. The use of metastable atom bombardment in ionizing the sample molecules has allowed the possibility of performing selective ionization, and control over the fragmentation of particles from the sample molecules. However, in order to perform metastable atom bombardment which consistently ionizes the sample material, a reaction mechanism is needed to produce a consistent source of metastable atoms, which is high in its intensity, charge free and low velocity.
A reaction system which produces a beam of metastable atoms is known in the art, and includes a reaction vessel having a source of rare gas at one end of the vessel, a cathode positioned inside the vessel and a small sonic nozzle placed at the other end of the vessel. Outside the vessel is a generally cone shaped anode referred to as a xe2x80x9cskimmerxe2x80x9d and which further includes an aperture at the apex of the cone. Behind the skimmer is a set of plates which serve as a deflector. In operation, the gas is injected at one end of the vessel and passes through the nozzle at the opposite end. The cathode within the vessel and the anode outside of the vessel are charged by a DC supply, such that a plasma arc is created between the cathode and anode. The atoms of gas which are injected through the discharge are energized to a metastable state, with some of the gas atoms being energized to the point of ionization, thus releasing free ions and electrons into the metastable gas stream. The metastable gas, the free ions and electrons then pass through the aperture in the apex of the skimmer into a set of charged deflector plates, where the free ions/electrons are attracted to the deflector plates, leaving the relatively charge free, metastable gas particles to pass through the deflector plates where it is used to bombard the sample substance to be analyzed by the mass spectroscopy apparatus.
A known disadvantage of this prior art device is that it does not always produce a consistent stream of metastable particles, and sometimes creates a stream of metastable particles mixed with ions/electrons. This occurs because the electric field which surrounds the cathode and anode is symmetric with respect to a longitudinal axis passing through the cathode and anode. As a result of this symmetric electrical field, the forces applied to the ions/electrons and ionized atoms created by the discharge is such that these particles are forced towards this longitudinal axis. Since this longitudinal axis also coincides with the axis of flow, the ions/electrons tend to remain in the flow path along with the metastable gas particles. Although the deflector does remove some of these ionized particles, the forces applied by the symmetric electric field work against the forces applied by the deflector, and thus ions tend to remain within the particle flow. Thus, the prior art apparatus does not produce a beam of purely metastable atoms, and produces spurious, unpredictable results when such a beam is used to ionize the sample to be tested by spectroscopy. The use of a skimmer and deflector plates also results in a larger assembly that causes a loss of metastable atoms. Because of the advantages of using metastable atom bombardment for selective ionization of the sample material, a need exists to improve the metastable atom bombardment system so that the beam of metastable atoms projected against the sample material only contains metastable atoms with a high density.
It is a feature of the present invention to provide an apparatus which efficiently produces a be am of metastable species having a good purity.
It is an other feature of the present invention to provide a method of generating a beam of purely metastable species for use in spectroscopy applications.
According to a first aspect of the present invention, the electric arc used to generate metastable gas follows a curved path.
According to a second aspect of the present invention, the gas subjected to the electric arc passes from a low pressure chamber through a nozzle to a lower pressure chamber to form a jet of gas, in which the jet of gas is subjected to fields for removing ionized gas from the jet of gas prior to a substantial portion of the jet exiting the lower pressure chamber as a pure metastable jet into a reaction chamber of a mass spectrometer. The intensity of the arc may be selected to generate a higher concentration of ionized and metastable species, while the jet exiting the lower pressure chamber comprises substantially only metastable species of the gas.
According to a third aspect of the present invention, the arc has a greater portion of its length in a higher pressure chamber than in the lower pressure chamber on the other side of the nozzle communicating between the higher and lower pressure chambers, so as to expend more energy in the higher pressure chamber.
According to one embodiment of the present invention, there is provided an apparatus for generating a beam of metastable species for use in Penning ionization, comprising:
first chamber having a gas inlet and a nozzle outlet, said inlet being connected to a substantially low pressure source of gas suitable for being energized to a metastable state and inducing Penning ionization and Penning energy transfer;
a cathode arranged in said first chamber;
a second chamber communicating with said nozzle and having a beam outlet substantially in line with said cathode and said nozzle, said second chamber being in communication with a substantially rough vacuum;
an anode arranged in said second chamber to one side of a line extending substantially between said cathode, said nozzle and said beam outlet, wherein an electrical discharge formed between said cathode and said anode passes through said nozzle and then deviates from said nozzle to said anode, and an electric field between said cathode and said anode is asymmetric,
whereby a jet of said gas emitted from said nozzle containing metastable and ionized species is projected to said beam outlet while ionized species are diverted from said beam outlet and a beam of said gas emitted from said beam outlet has an improved concentration of metastable species.
The invention also provides method of generating a beam of metastable atoms for use in Penning ionization, comprising the steps of:
providing a jet of gas suitable for being energized by electrical discharge to a metastable state and inducing Penning ionization;
forming a curved electrical discharge arc co-extensive with a portion of the jet and deviating from the jet to one electrode to excite the gas to a metastable state; and
communicating a downstream portion of the jet with a beam outlet.
The invention further provides a method of ionizing and fragmenting a molecule, the method comprising the steps of:
selecting a gas having an energy of a metastable state sufficient to cause ionization in the molecule and to break at least one desired bond in the molecule;
generating a beam of the gas excited to the metastable state, the beam being substantially free from ions;
providing the molecule is a gaseous state in an ionization reaction chamber; and
directing the beam into the reaction chamber to cause ionization and selective fragmentation of the molecule.